1. Field of the Invention
The present invention relates generally to an automatic control circuit for controlling the depth of focus of a video shooting apparatus and, more particularly to an automatic control circuit for controlling the depth of focus by automatically controlling the quantity of light of an object in dependence upon a shutter speed for a special-shooting.
2. Description of the Prior Art
In general, video shooting apparatuses are camcorders or video camera, electronic still camera etc. Particularly, a video camera is a device for converting an image of an object (scenery, figure etc.) into an electrical signal (an image signal) and is called a TV camera or a telecamera. If a person shoots an object by such a video camera, at first, an image is electrically converted from an image element and is output through a sample and hold circuit to provide a peak value or an average value to be applied to an iris detector for controlling the opening and closing of an iris of lens in dependence upon an incidence of the suitable quantities of light to a camera according to the state of light quantities (illuminance; Lux). The depth of focus is determined according to the degree of the opening and closing of such iris. The depth of focus is used to adjust a focal point by moving a focal plane in the front and rear directions within an option distance without moving the position of an object. Accordingly, the more the iris of lens is tighten, the more the depth of focus is deepened. Then, as the depth of focus is equal to the depth of background within a constant distance centering on an object as well as an object, in a conventional art, the depth of focus has been controlled by attaching a centerfocus optical filter to the front plane of lens on special-shooting such as a centerfocus centering on an object. Therefore, it is a tremendous convenience to a user since the conventional art requires that the centerfocus optical filter to be attached every time a centerfocus is desired.